Manganese Removal by the Epilithic Microbial Consortium at Pinal Creek near
Globe, Arizona

ABSTRACT

Interaction of an acidic mine drainage plume with subsurface material in an
alluvial aquifer has released dissolved manganese [(Mn(II)] into the perennial
reach of Pinal Creek near Globe, Arizona. A combination of hydrologic and biogeochemical
precesses is responsible for precipitation of a fraction of the entering Mn(II)
as Mn-oxyhydroxides on surficial sediments, within the streambed sediments,
beneath algal mats formed on surficial sediments, and among mosses and emergent
aquatic plants. This study focuses on the variety and seasonality of biological
processes associated with Mn-oxyhydroxide precipitates formed on glass substrates
placed in surface waters characterized by different flows and vegetation. The
glass slides were emplaced monthly at a single subreach of Pinal Creek to assess
epilithic attachment and Mn oxidation; epiphytic oxidation was assessed periodically
also. Oxidized Mn was associated with almost every organism in the consortium
at Pinal Creek, from the microscopic to the macroscopic. Epilithic bacteria,
fungi, algae, and protozoans were coated with oxidized Mn; every macrophyte
examined had patches of oxidized Mn. The dominant epilithic precipitation forms
were around holdfasts and within secreted substances. The black holdfasts of
the iron bacterium, Leptothrix discophora, and the green alga, Ulothrix sp.,
were doughnut-shaped forms. Expansive patches of black extracellular polysaccharides
were secreted primarily by bacterial filaments and fungal hyphae. The dominant
macrophytic precipitation form was clumps of oxidized Mn on mosses, green algae,
and cyanobacteria. These clumps are consistent with Mn precipitation by elevated
pH during photosynthesis. More Mn-oxide precipitates were found in the spring
and summer months than the fall and winter, consistent with biological and chemical
activity models, and more formed in swifter water than in slower moving water,
consistent with oxygen elevation models. These findings provide a better understanding
of the biological factors that influence natural attenuation of Mn at Pinal
Creek and identify some of the complex interactions between biota, hydrologic
processes, and water chemistry that need to be considered to fully assess the
affects of acidic mine drainage on stream systems.